Power distribution cables are provided with an outer sheath of insulating material and it is common practice to check the insulation of the outer cable sheath before the cable is put into operation. Normally, a test voltage of e.g. between 5 and 15 kV is used to apply an electric potential across the outer sheath in order to verify that its insulating properties are intact. In practice, for underground cables, this is done by applying the electric potential between a screen located underneath the outer sheath and the ground surrounding the cable.
In installations where the power cables are not laid under ground or are laid e.g. in a plastic tube it has been a recent and quite frequent requirement that cables shall have an outermost conducting layer or jacket that is applied to the insulating outer cable sheath surrounding the cable core or cores. In fact, it is generally desirable to provide such outer conducting jackets also in most underground installations, where ground contact may often be inferior due to air pockets around the cable or due to very dried out ground surrounding the cable.
The purpose of such conducting jackets is to enable the described checking of the insulation of the outer cable sheath in the absence of the surrounding ground or by the described conditions of inferior ground contact. It is thus common practice to provide round, generally circular-section power cables with such an outermost conducting jacket surrounding the outer periphery of the cable. This jacket is firmly interconnected with the sheath material and is in fact normally extruded simultaneously with the sheath material in a double extruder. A short length of such an outermost conducting jacket will have to be removed at cable joints and cable terminations to avoid flashover at the edge of the insulating sheath.
Outer conducting jackets of the existing types are normally not used for power cables having a non-circular cross section, such as the common 3 core cables. Since milling tools or similar tools requiring a round surface can not be used for such non-round cables it would be very difficult and time consuming to effectively remove such outermost conducting jackets from them at cable joints and terminations. Accordingly, today there is no method available for the effective, rational removal of an outer conducting jacket from such non-round cables.
The removal of an outermost conducting jacket is not without problems even for generally round cables and may thus have consequences. It is normally performed by means of the above indicated type of milling or other tools that can be used for cables having a generally circular cross section. However, a drawback of using such tools is that they may leave residue of the conducting jacket. It is often difficult to visually determine that all conducting material has been effectively removed and remaining residue may cause problems with regard to accurately performing the described insulation check voltage test. Such conducting layer residue could potentially also cause problems like the above mentioned risk of flashover at cable joints and terminations. It has therefore been suggested to produce the conducting jacket in a color that is easily distinguishable from that of the standardized black outer insulating sheath. This is an unwanted complication since the conducting jacket will normally be black for the technical reason of achieving the required electrical conductivity.